1. Field of the Invention
This invention relates to a glass polarized lens, more particularly to an edge coated glass polarized lens having a peripheral edge coated with an edge coating.
2. Description of the Related Art
The use of polarized lenses becomes increasingly popular in sunglasses. More than regular sunglass lenses, polarized lenses provide additional protection from annoying glare without a very dark tint. Glare is produced when bright light is reflected from smooth surfaces, such as water, snow, and roadways. Glare not only causes ocular fatigue but also creates serious problem to driving safety in terms of limited vision under the presence of blurring glare.
The most common structure of a polarized lens is a multiple-layer lens system comprising at least a first lens wafer, a second lens wafer, and a polarizing layer laminated between the first and second lens wafer. The material of a lens wafer can be glass or plastic. Glass lens material usually performs better than their plastic counterpart on scratch resistance, chemical resistance, optical quality, and polarization efficiency. On the contrary, plastic lens material is more light in weight and impact resistant.
One big challenge a multiple-layer polarized lens will face is delamination. The term delamination refers to separation happened between any two layers of a polarized lens system. Possible causes of lens delamination include the heat and vibration applied to a lens edge during lens edging process, the stress received by a lens edge during and after framing process, and the penetration of water when a laminated lens is used in hot and humid conditions like water sports.
Among those causes described above, the delamination induced by framing stress is the most troublesome one because it may not be detected in the manufacturing process. Very likely a glass polarized lens will delaminate if it is glazed into a frame in a hard way, particularly if the lens is too big in size or unfitted in shape for the frame to glaze. Abnormally big framing stress will be created on the sharp lens bevel apex and delaminate the lens. The initial delamination in this type is usually fine and not visible to inspecting people if inspection is done in just 1 or 2 days from framing. After framing, the tight frame keeps exerting pressure on lens edge and enlarging delaminated area until the additional stress is released through delamination. By that time it would be too late for a manufacturer to find out the defective sunglass because it may be packed or even sent to a customer.
In search of a solution to the delamination problem encountered by a glass polarized lens, edge coating is a sound method because it provides additional binding force to a lens edge against the delamination induced by framing stress. Edge coating also helps to seal a lens edge to inhibit delamination induced by the penetration of water.
U.S. Pat. No. 4,076,863 discloses an edge coating material selected from epoxy resins, alkyd resins, polyurethane resins, unsaturated polyester resins, acrylic resins, and cyanoacylates. Likewise, U.S. Pat. No. 5,220,358 discloses an edge coating including a dimethyl silicone coblocked with vinyl. In both cases, the coating materials need to be cured at a temperature 100° C. or higher for at least 30 minutes. This curing condition will change the color of a polarized lens too much to be cosmetically acceptable since the color of a polarizing layer of a polarized lens is very sensitive to heat above 50° C. A curing temperature lower than 50° C. may prevent such an unfavorable change to a lens color; however, the curing process will become too time consuming. U.S. Pat. No. 6,860,600 discloses a method of edge coating a lens wherein an edge coating material that will strongly bond to the edge but only weakly bond to the optical surfaces of a lens is selected and applied to form a durable edge coating. However, the edge coating materials comprising essentially acrylate solved in organic solvent as set for examples in the method show a poor adhesion with the edge of a glass polarized lens in physical tests. It separated from almost all around the edge of a glass polarized lens after framing. Once an edge coating was separated from a lens edge, it would no longer provide a lens edge with additional binding force against delamination induced by framing stress. Neither can it bean effective protection to a lens edge against delamination induced by the penetration of water. Meanwhile, most acrylates are known of causing sever skin irritation; therefore, excluded from ink formulations by the Society of British Printing Ink Manufacturers (SBPIM). The use of organic solvent is also considered not healthy or environment friendly.
There is a need for edge coating a glass polarized lens that overcomes problems with known methods, and it is the aim of the present invention to provide such a method.